What people usually do is buy one now and for a cheap performance boost buy another down the line. Nvidia have taken this option away. If people in this bracket want an upgrade they will have to buy a more expensive card or buy again when the next gen arrives.
Fair enough, but I've always seen that as uneconomical as performance varies between games with SLI. I would rather sell the 1060 after 18 months and buy a 1070 if I wanted an upgrade.
Buy 1060 for $300, then sell for $150 18 months later, then buy 1070 for $449. Or buy 1060 for $300, then buy another for $300 18 months later. Same money, but I would definitely rather have a single card.